The present invention relates to a screw rotor in a spiraxial screw rotor type vacuum pump.
The spiraxial screw rotor type vacuum pump is a pump in which a pair of right-hand screw rotor and left-hand screw rotor having a plurality of threads (number of leads) are engaged with each other and rotate in a casing. Because such a pump has high pumping efficiency, it has been used not only as an ordinary pump for supplying a fluid but has been widely used as a vacuum pump.
As shown in FIG. 4, a shape of a screw rotor A in a section perpendicular to its axis is composed of a Quimby curve a, a circular arc b of a thread bottom, a quasi-Archimedean spiral curve c and a circular arc d of an outer diameter. Since a center of gravity G of the rotor is positioned eccentrically from a rotation axis of the screw rotor in a direction of xcfx86, an unbalanced mass is created at rotation.
Meanwhile, in order to minimize the screw rotor type vacuum pump, it is necessary to rotate the screw rotor at a high speed. Because leaking amount of the fluid to be supplied is considered to be constant irrespective of rotation number, and the higher the rotation speed becomes, the more capacity efficiency will be enhanced, it is desired to make the screw rotor rotatable at the high speed. However, due to the unbalanced mass of the screw rotor as described above, vibrations will occur with the high speed rotation.
In view of the above, in order to use the screw rotor type vacuum pump with high efficiency taking its advantages into account, the screw rotor type vacuum pump must be such that it can be used even in a high speed rotation range. For this reason, it is necessary to improve the rotor balance to reduce the vibrations which occur in the high speed rotation range. For this purpose, the applicant has proposed in Japanese Patent Publication No. 2-17716 a screw rotor in which a static balance is set by making the leads of the threads in the screw rotor to be an even number, and a dynamic balance is set by providing cavities in thread ridges on both ends of the screw rotor, or by casting a light material into the cavities.
In order to set the dynamic balance by providing the cavities in the thread ridges on both the ends of the screw rotor, a core for forming the cavities must be large and rigid. In case where the core is small and fragile in shape, there will be created a core displacement or a crack when the rotor is cast, and precise cavities cannot be formed.
In the meantime, as a demand for a smaller sized vacuum pump is prevailing in the market, if a lead length L as shown in FIG. 5A is reduced to a lead length Lxe2x80x2 as shown in FIG. 5B, a thread e is reduced in thickness to form a thin thread exe2x80x2.
It is difficult to produce small cavities in the thin thread exe2x80x2 by casting, and it has been only means to set the balance by drilling.
Even by drilling, only a small unbalanced amount can be removed from the thin thread exe2x80x2, and therefore, the unbalanced amount must be as small as possible.
The applicant has conducted tests varying total lengths of the threaded portion of the screw rotor, and found that an unbalanced moment of the screw rotor becomes least when the total length of the threaded portion is under a determined condition.
The present invention has been made on the basis of the result of the tests, and an object of the invention is to provide a screw rotor for a vacuum pump which has the least unbalanced moment.
In order to attain the above described object, there is provided a spiraxial screw rotor type vacuum pump, in which a pair of right-hand screw rotor and left-hand screw rotor, a section perpendicular to each axis of which is asymmetrically formed by a Quimby curve, a circular arc and a quasi-Archimedean spiral curve, are engaged with each other, characterized in that number of thread leads of threaded portions in the screw rotors is (an integer +0.5), thereby to decrease unbalanced moments of the screw rotors.